1. Field of the Invention
The present invention relates to corneal repair, and more in particular, to a novel keratoprosthesis that provides enhanced soft tissue adhesion and less extrusion, a system for repairing severely damaged cornea using such keratoprosthesis, and a method of implanting such keratoprosthesis to effect corneal repair.
2. Description of the Related Art
One of the greatest challenges in ophthalmology is the management of the severely damaged cornea. Severe damage to the cornea can arise from Stevens Johnson's syndrome, chemical burns, Cicatricial pemphigoid, Lyell's syndrome and recurrent graft failure, for example. These conditions can lead to scarring of the cornea and corneal blindness that is not amenable to treatment or repair by common techniques such as corneal transplantation (also called “penetrating keratoplasty”). In these circumstances, management is limited to a keratoprosthesis.
The three most frequently used keratoprostheses currently are the autogenous modified osteo-odonto keratoprosthesis (MOOKP), the alloplastic Boston Keratoprosthesis®, and the AlphaCor™ keratoprosthesis. The Alphacor™ keratoprosthesis was abandoned in the United States due to poor results after two years, mainly caused by retraction of the cornea at the interface with the prosthesis which can cause aqueous leaks and consequent loss of intraocular pressure and potential endophthalmitis. The Boston Keratoprosthesis® has had better results but also suffers from cornea thinning and retraction at the cornea-implant interface which, if not repaired in time, can also lead to extrusion, or rejection of the implant, and endophthalmitis. Also, the Boston Keratoprosthesis® requires a donor cornea. Human tissue availability might be problematic in countries where donor tissue is difficult to obtain or not available. In addition, as the Boston Keratoprosthesis® has a fixation member, a plastic or a metal ring, located in the anterior chamber of the patient, contact with the iris is problematic as when the patient's iris closes in bright light, it may contact the fixation ring or the protruding posterior surface of the implant and iris inflammation occurs. The inflammation can induce fibrinous secretion which engenders the formation of an opaque intraocular fibrous capsule (called retroprosthetic membrane) which impedes vision and needs intraocular surgery to be resected. The fibrous membrane sometimes closes the anterior chamber angle preventing normal aqueous outflow and the intraocular pressure rises. If the membrane is not removed surgically, the increase in intraocular pressure may cause glaucoma and loss of vision. The major problem with all alloplastic keratoprostheses is the lack of tissue attachment, and therefore a potential rejection.
Of the current keratoprostheses, the MOOKP has had the most success thus far, having been used for over 30 years with a documented success rate of approximately 85%. Generally, the MOOKP comprises a polymethylmethacrylate (PMMA) optic cylinder embedded into an autogenous graft consisting of tooth, periodontal ligament (PDL), bone, and periosteum. The implantation procedure is a multistep process involving both maxillofacial and ocular surgery. The first step includes preparing the eye by cutting and removing the synechiaes to free the globe, removing all corneal and conjunctival epithelium, and removing the crystalline lens and iris. The second step includes the excision and transfer of oral buccal mucosa from the patient's mouth to the anterior surface of the eye. A combination of tooth and bone is then resected from the patient and formed into a support skirt of approximately 3 millimeters in thickness. The selection of a tooth used in the MOOKP procedure is based on clinical and radiographic evaluation of the patient's dentition. The surgeon selects the largest tooth root, usually the maxillary canine, for use in the MOOKP. The largest root allows for the optical cylinder with the largest diameter to be utilized for the prosthesis. Traditionally, the optical cylinder varies in diameter between three and four millimeters. The larger the cylinder, the wider the field of view for the patient, therefore the largest optical cylinder (determined by the width of the patient's chosen canine, which varies patient-to-patient) that can be fitted into the support skirt, or lamina, without causing structural damage is selected. A hole is then hand-drilled in the support skirt to fit the selected optical cylinder, which is then cemented into the skirt to form the MOOKP. The skirt is then implanted into a subcutaneous pocket within the cheek or subclavicular chest wall of the patient. After a three month healing period, which allows the formation of a neovascular network of tissue and blood vessels to attach to the prosthesis, the third step of the procedure involves transplanting the prosthesis, along with the newly attached tissue, from the subcutaneous pocket to a pocket created deep to the transplanted buccal mucosa over the host cornea. The de novo tissues attached to the implant are then sutured to the surrounding environment to keep the prosthesis in place.
Despite its relative success, the MOOKP suffers from multiple complications and drawbacks. For example, ocular complications can develop subsequent to implantation, including glaucoma, infection, extrusion, and retinitis. In fact, a primary cause of keratoprosthesis failure is extrusion, in which epithelial cells infiltrate the implant area and literally push the implant out. Secondary infections resulting from bacterial invasion upon epithelial infiltration is also a major problem contributing to prosthesis rejection. Dental complications arising from the resection of the osteo-odonto lamina may produce damage to adjacent teeth, oro-nasal and oro-antral communication, infection, sinusitis, and nerve damage. Drawbacks of the MOOKP procedure include a poor cosmetic outcome of the eye. Although a shell can be developed to cover the buccal mucosa in an attempt to improve the cosmesis of the eye, often times the osteo-odonto lamina or skirt displaces such a large volume of space that the eye cannot be appropriately covered. Therefore, the appearance of the eye cannot be fully or completely improved. In another drawback, the MOOKP procedure creates a surgical defect of the oral cavity from the removal of tooth and bone. This defect ideally can be reconstructed utilizing a bone graft followed by a dental implant or additional options of dental rehabilitation. However, such an oral defect is complex and generally is difficult to reconstruct. Finally, the multistage nature of the MOOKP procedure, involving two different groups of surgeons, and the duration of the various steps makes it an inherently prolonged procedure with numerous opportunities for complications to arise.
A new keratoprosthesis is therefore needed which emulates the success of the MOOKP without the resultant complications and defects. The ideal keratoprosthesis should have optimal biointegration, resist infection, replicate qualities of the cornea including drug penetration and intraocular pressure measurements, and last the lifetime of the patient. The keratoprosthesis should also prevent, or at least reduce, the risk of extrusion and rejection of the keratoprosthesis as well as iris inflammation.